Sine wave generation circuit and uninterruptible power supply system using the same

ABSTRACT

A sine wave generation circuit and an uninterruptible power supply system (UPS) using the same, and more particularly, a sine wave generation circuit and a UPS using the same, which can output a sine wave for direct current (DC) power from a battery. In the UPS, a rectifier rectifies commercial alternating current (AC) power from an input terminal and converts the AC power into DC power. A charger charges a battery with the DC power. The battery provides the DC power. A DC-DC converter boosts and/or drops the DC power inputted from the battery by a predetermined level of the AC power. A D-class amplifier receives the DC power from the DC-DC converter and outputs a sine waveform power signal in response to a waveform control signal. A sine waveform controller controls a sine waveform generation operation of the D-class amplifier. A switching unit switches the commercial AC power from the input terminal to a load in a normal mode, and switches the sine waveform power signal from the D-class amplifier to the load when an error of the commercial AC power is detected.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a sine wave generation circuitand an uninterruptible power supply system (UPS) using the same, andmore particularly to a sine wave generation circuit and anuninterruptible power supply system (UPS) using the same, which canoutput a sine wave for direct current (DC) power stored in a battery.

[0003] 2. Description of the Related Art

[0004] All electronic products using commercial power receive sine wavesin order to operate normally. Where an electronic product is implementedas a mobile electronic product or backup equipment, electric powercorresponding to the commercial power must be provided to the electronicproduct. Mobile electronic products using commercial alternating current(AC) power include commercial laser and light equipment (100 W-500 W),measurement equipment (100 W-1 KW) installed in a special vehicle forperforming various inspections, a bus-dedicated television orrefrigerator (100 W-1 KW), etc. Furthermore, the backup equipmentincludes an uninterruptible power supply system (UPS) (500 W-3 KW), afrequency converter (500 W-1 KW), a solar-cell power converter (1 KW-5KW), etc.

[0005] As described above, the mobile electronic product and backupequipment using the commercial AC power are commonly equipped with abattery, convert direct current (DC) power stored in the battery intothe commercial AC power, and provide the commercial AC power to a load(or a main body of an electronic product).

[0006] However, an inverter (e.g., an AC-DC converter) provided in themobile electronic product mostly outputs a modified square wave. Thus,the inverter can cause a serious damage to an electronic productequipped with a motor. In addition, the inverter is large and heavy.

[0007] Since a converter (e.g., a DC-DC converter) or inverter for usein the backup equipment uses an inductance and capacitance (LC) resonantcircuit to generate an AC wave, the efficiency of the converter orinverter is very low. Furthermore, since the converter or inverter foruse in the backup equipment is large and heavy, its mobility is limitedand its price is high.

[0008] On the other hand, an accident due to a power failure frequentlyoccurs recently in comparison with the past. The amount of damage due tothe power failure at the present is more considerable in comparison withthat at the past. The UPS is a system for supplying high-qualityelectric power to prevent an outage or malfunction of a facility due toa sudden power supply failure associated with an instantaneous powerfailure or an instantaneous voltage or frequency variation in highlyadvanced computers, plant monitoring control devices, communicationequipment, plant process control devices, and major equipment for use ina hospital, etc. The UPS provides electric power during a power outageusing a battery to prevent damage due to the above-described powerfailure, to prevent an undesired outage of industrial machines, and toeliminate the inconvenience in using the Internet and in daily life.

[0009] According to a structure of the UPS, the UPS includes amechanical UPS and an electronic UPS. The mechanical UPS stores electricenergy when electric power is supplied normally, rotates a motor with DCvoltage stored in the battery at the time of a power failure, drives agenerator with the rotating force, and produces commercial AC voltage.The electronic UPS consecutively converts the battery's voltage in apredetermined period to produce AC voltage.

[0010] In the case of the mechanical UPS, a waveform of the AC voltagemust be identical to that of the commercial AC voltage. When a devicereceiving electric power is an inductive load, a motor brush can bedamaged due to ark generation where the AC power supply cannot maintaina 60 Hz sine wave. If voltage induced through a transformer isinappropriately increased, a load device can be damaged. Furthermore,since the mechanical UPS is large and heavy and its efficiency is low,it is difficult for the mechanical UPS to be used in small-sizedequipment or indoors.

[0011] In the case of the electronic UPS, its circuitry is complex. Theelectronic UPS is expensive and frequently failed because it must drivethe circuit at a high speed. Accordingly, an improved method used bymost UPSs cheaply produces a square wave, produces AC power (based onthe square wave) through a linear transformer, removes values ofstarting and end points of the square wave using an inductance andcapacitance (LC) resonant circuit, etc., and produces and uses a wavesimilar to a sine wave. Since this improved method cannot produce a trueAC wave, a motor can be seriously damaged. The above-describedelectronic UPS has low efficiency and is large and heavy. Furthermore,the conventional UPSs cannot be applied to mobile products.

[0012] The conventional UPSs have been fixed in the past. However, theUPSs need to be carried to various locations since the electric power isused not only in industrial machines but also in daily life. To do this,the UPSs must be lightened in weight and slimmed in size. Of course, theUPSs must functionally supply a 60 Hz sine wave as in a commercial powersupply.

[0013]FIG. 1 is a view illustrating an exemplary conventionaluninterruptible power supply system (UPS).

[0014] A rectifying/charging unit 101 rectifies commercial alternatingcurrent (AC) power when the commercial AC power is supplied, convertsthe rectified commercial AC power into direct current (DC) power, andcharges a battery 103 with the DC power.

[0015] An inverter 105 receives the DC power from therectifying/charging unit 101 and the battery 103 connected to therectifying/charging unit 101 in parallel. Furthermore, the inverter 105generates AC power in the form of a square wave in response to a pulsewidth modulation (PWM) control signal from a PWM generator (not shown inFIG. 1).

[0016] A linear transformer 107 boosts a level of AC power applied fromthe inverter 105, and an inductance and capacitance (LC) resonantcircuit 109 converts the AC power boosted by the linear transformer 107into the form of a sine wave by removing peak values of starting and endpoints of a square wave.

[0017] For reference, circuits of the inverter 105 and the lineartransformer 107 can be configured as shown in FIG. 7.

[0018] If the commercial AC power is appropriate, a bypass switch 111applies the commercial AC power to a load. On the other hand, if thecommercial AC power is inappropriate, auxiliary power processed by thesystem elements indicated by the reference numerals 101 to 109 isapplied to the load.

[0019] As described in relation to FIG. 1, the conventional UPSconfigures a filter circuit such as an inductance and capacitance (LC)resonant circuit to generate a sine wave. The conventional UPS cannotcorrectly generate the sine wave. Thus, the efficiency of theconventional UPS is low. The conventional UPS is large in size and heavyin weight. Furthermore, the conventional UPS cannot be appropriatelyapplied to a mobile product.

[0020] As the linear transformer 107 is employed when the DC power ofthe battery 103 is boosted, there are problems in that the efficiency ofthe conventional UPS is low, and the conventional UPS is large in sizeand heavy in weight.

[0021] On the other hand, a digital amplifier is an amplifier fordigitally amplifying an analog source signal. After converting theanalog source signal into a pulse width modulation (PWM) signal, thedigital amplifier carries out an amplifying operation for the PWMsignal. The PWM signal indicates a digital signal of one bit, and anaudible signal level associated with the PWM signal is recorded assignal width. An amplifying stage for amplifying the PWM signal as atype of switch does not affect straight wave characteristics associatedwith a transistor. The PWM signal is filtered by a low-frequency filter,and is restored to an original analog signal. The digital amplifier orD-class amplifier restores the original analog signal using thelow-frequency filter after amplifying a digital signal using the PWMamplifier.

[0022] Where the D-class amplifier is used, a desired output wave can bereproduced and outputted. Thus, where the D-class amplifier is appliedto the mobile electronic product or backup equipment such as anelectronic product using commercial AC power, DC power stored in thebattery is converted into a waveform (or sine waveform) signal ofhigh-quality commercial AC power, and the high-quality commercial ACpower can be applied to a load.

[0023] The basic concept of the digital amplifier has been known sincethe 70's, but technology of the D-class amplifier was not applied to thefield in the invention. The reason is that high fidelity performance ofthe digital amplifier can be implemented only when a digital signalprocessing (DSP) algorithm for converting a pulse code modulation (PCM)code into a PWM signal, technology for designing a high-speed DSPapplication specific integrated circuit (ASIC) operating at 100 MHz orabove, and all technologies of electronic and information industrialfields for digitally switching and amplifying a low-power PWM signal toa high-power PWM signal are entirely harmonized. For this reason, noamplifier having the high fidelity performance has been commercializedup to now.

SUMMARY OF THE INVENTION

[0024] Therefore, the present invention has been made in view of theabove problems, and it is one object of the present invention to providea sine wave generation circuit and an uninterruptible power supplysystem (UPS) using the same, which can generate a true sine wave througha switching method for direct current (DC) power stored in a battery toapply the generated true sine wave to a load.

[0025] It is another object of the present invention to provide a sinewave generation circuit and an uninterruptible power supply system (UPS)using the same, which can have a small volume and light weight.

[0026] It is another object of the present invention to provide a sinewave generation circuit and an uninterruptible power supply system (UPS)using the same, which can directly output a true sine wave through aswitching method based on a digital amplifier theory.

[0027] In accordance with one aspect of the present invention, the aboveand other objects can be accomplished by the provision of anuninterruptible power supply system (UPS), comprising: a rectifier forrectifying commercial alternating current (AC) power from an inputterminal and converting the AC power into direct current (DC) power; acharger for charging a battery with the DC power; the battery forproviding the DC power; a DC-DC converter for boosting and/or droppingthe DC power inputted from the battery by a predetermined level of theAC power; a D-class amplifier for receiving the DC power from the DC-DCconverter and outputting a sine waveform power signal in response to awaveform control signal; a sine waveform controller for controlling asine waveform generation operation of the D-class amplifier; and aswitching unit for switching the commercial AC power from the inputterminal to a load in a normal mode, and switching the sine waveformpower signal from the D-class amplifier to the load when an error of thecommercial AC power is detected.

[0028] Thus, the present invention enables a correct sine waveform to bedirectly generated through only a control operation for the D-classamplifier having a low-speed switching period and a cheap microcomputerwithout use of a high-speed analog/digital (A/D) converter, a switchdevice, a digital signal processor (DSP) and a digital/analog (D/A)converter, such that a sine wave generation circuit for directlygenerating a true sine wave from DC power stored in a battery through aswitching method and applying the generated true sine wave to the loadand an uninterruptible power supply system (UPS) using the same can beappropriately implemented. Further, as an electronic product and backupequipment using the commercial power can be miniaturized and lightenedin weight, a mobile electronic product can be implemented. Furthermore,as the true sine wave (or correct sine waveform) can be directlygenerated through the switching method and the generated true sine wavecan be applied to the load, a product can be appropriately protected.

[0029] Preferably, the D-class amplifier may comprise a bridge circuitfor power conversion. Preferably, the bridge circuit may comprise: afirst inductance device (L1) for high-frequency pass arranged on a path(A-L2) between a second inductance device (L2), arranged on a path forconnecting a first node (A) and a second node (B), and the first node(A); a third inductance device (L3) for high-frequency pass arranged ona path (L2-B) between the second inductance device (L2) and the secondnode (B); a first capacitance device (C1) including one end thereofconnected to a third node (C) arranged on a path (L1-L2) between thefirst inductance device (L1) and the second inductance device (L2), andthe other end thereof connected to a ground side; a second capacitancedevice (C2) including one end thereof connected to a fourth node (D)arranged on a path (L2-L3) between the second inductance device (L2) andthe third inductance device (L3) and the other end thereof connected tothe ground side; and two load output terminals (X and Y) connected toboth ends of the second inductance device (L2). Preferably, the sinewaveform controller may perform a control operation so that a differencebetween a turn-on time of one pair of switching devices (SW1 and SW4)provided in the D-class amplifier and a turn-on time of the other pairof the switching devices (SW2 and SW3) provided in the D-class amplifiercan be generated, and an output terminal (X or Y) can output voltage ofthe turn-on time difference every time a predetermined switching periodis shorter than a commercial AC power period. Preferably, the sinewaveform controller may adjust the turn-on time difference in eachswitching period, and perform a control operation so that the voltageoutputted through the output terminal (X or Y) corresponds to sinewaveform power equal to the commercial AC power.

[0030] In the UPS, the D-class amplifier can be easily applied to acircuit for converting the DC power stored in the battery into awaveform signal of commercial AC power and applying the converted powerto the load.

[0031] In accordance with another aspect of the present invention, theabove and other objects can be accomplished by the provision of a sinewave generation circuit for converting a waveform of direct current (DC)power stored in a battery into a waveform of commercial alternatingcurrent (AC) power and applying the commercial AC power, comprising:

[0032] a bridge circuit for sine wave generation, the bridge circuitcomprising:

[0033] a first switching device (SW1) for receiving the DC power;

[0034] a second switching device (SW2) for receiving the DC power;

[0035] a fourth switching device (SW4) connected to the first switchingdevice (SW1) through a path (A->B) between a first node (A) and a secondnode (B);

[0036] a third switching device (SW3) connected to the second switchingdevice (SW2) through a path (B->A) between the second node (B) and thefirst node (A);

[0037] a second inductance device (L2) arranged in the path (A-B)between the first node (A) and the second node (B);

[0038] a first inductance device (L1) for high-frequency pass arrangedin a path (A-L2) between the first node (A) and the second inductancedevice (L2);

[0039] a third inductance device (L3) for high-frequency pass arrangedin a path (L2-B) between the second inductance device (L2) and thesecond node (B);

[0040] a first capacitance device (C1) including one end thereofconnected to a third node (C) arranged on a path (L1-L2) between thefirst inductance device (L1) and the second inductance device (L2), andthe other end thereof connected to a ground side;

[0041] a second capacitance device (C2) including one end thereofconnected to a fourth node (D) arranged on a path (L2-L3) between thesecond inductance device (L2) and the third inductance device (L3) andthe other end thereof connected to the ground side; and

[0042] two load output terminals (X and Y) connected to both ends of thesecond inductance device (L2); and

[0043] a sine waveform controller for applying a sine wave generationcontrol signal to the switching devices (SW1˜SW4), and performing acontrol operation so that one pair of the first and fourth switchingdevices (SW1 and SW4) and the other pair of the second and thirdswitching devices (SW2 and SW3) can alternately perform aturn-on/turn-off operation, wherein the sine waveform controllerperforms a control operation so that a difference between a turn-on timeof one pair of switching devices and a turn-on time of the other pair ofswitching devices can be generated, and an output terminal (X or Y) canoutput voltage of the turn-on time difference every time a predeterminedswitching period is shorter than a commercial AC power period, and

[0044] wherein the sine waveform controller adjusts the turn-on timedifference in each switching period, and performs a control operation sothat the voltage outputted through the output terminal (X or Y)corresponds to sine waveform power equal to the commercial AC power.

[0045] In the UPS, the D-class amplifier can be easily applied to acircuit for converting the DC power stored in the battery into awaveform signal of the commercial AC power and applying the convertedpower to the load. Furthermore, the present invention enables a correctsine waveform to be directly generated through only a control operationfor a D-class amplifier having a low-speed switching period and a cheapmicrocomputer without use of a high-speed analog/digital (A/D)converter, a switching device, a digital signal processor (DSP) and adigital/analog (D/A) converter, such that a sine wave generation circuitfor directly generating a true sine wave from DC power stored in thebattery through a switching method and applying the generated true sinewave to the load and an uninterruptible power supply system (UPS) usingthe same can be appropriately implemented. Further, as an electronicproduct and backup equipment using the commercial power can beminiaturized and lightened in weight, a mobile electronic product can beimplemented. Furthermore, as the true sine wave (or correct sinewaveform) can be directly generated through the switching method and thegenerated true sine wave can be applied to the load, a product can beappropriately protected.

[0046] The sine wave generation circuit may further comprise a DC-DCconverter for boosting and/or dropping the DC power inputted from thebattery by a predetermined level of the AC power and inputting theboosted or dropped DC power into the bridge circuit for sine wavegeneration.

[0047] Since the present invention uses a switching transformationmethod without using a linear transformation method and an inductanceand capacitance (LC) resonant circuit provided in the conventional UPS,the UPS can be miniaturized and lightened in weight where the sine wavegeneration circuit is applied to the UPS.

BRIEF DESCRIPTION OF THE DRAWINGS

[0048] The above and other objects, features and other advantages of thepresent invention will be more clearly understood from the followingdetailed description taken in conjunction with the accompanyingdrawings, in which:

[0049]FIG. 1 is a view illustrating an exemplary conventionaluninterruptible power supply system (UPS);

[0050]FIG. 2 is a view illustrating the configuration of a D-classamplifier in accordance with the present invention;

[0051]FIG. 3 is a graph illustrating the characteristics of boostedvoltage across a capacitance device varying with time;

[0052]FIG. 4 is a view illustrating an operation for outputting sinewave power to a load;

[0053]FIG. 5 is a block diagram illustrating a connection structure inthe case where a sine wave generation circuit is applied to a mobileelectronic product or backup equipment using commercial alternatingcurrent (AC) power;

[0054]FIG. 6 is a block diagram illustrating an exemplary UPS equippedwith the sine wave generation circuit; and

[0055]FIG. 7 is a circuit diagram illustrating an inverter and a lineartransformer.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0056] Now, preferred embodiments of the present invention will bedescribed in detail with reference to the annexed drawings.

[0057]FIG. 2 is a view illustrating the configuration of a D-classamplifier in accordance with the present invention.

[0058] As shown in FIG. 2, the D-class amplifier of the presentinvention includes a first switching device SW1 for receiving directcurrent (DC) power stored in a battery (not shown); a second switchingdevice SW2 for receiving the DC power; a fourth switching device SW4connected to the first switching device SW1 through a path (A->B)between a first node A and a second node B; a third switching device SW3connected to the second switching device SW2 through a path (B->A)between the second node B and the first node A; a second inductancedevice L2 arranged in the path (A-B) between the first node A and thesecond node B; a first inductance device L1 for high-frequency passarranged in a path (A-L2) between the first node A and the secondinductance device L2; a third inductance device L3 for high-frequencypass arranged in a path (L2-B) between the second inductance device L2and the second node B; a first capacitance device C1 including one endthereof connected to a third node C arranged on a path (L1-L2) betweenthe first inductance device L1 for the high-frequency pass and thesecond inductance device L2, and the other end thereof connected to aground side; a second capacitance device C2 including one end thereofconnected to a fourth node D arranged on a path (L2-L3) between thesecond inductance device L2 and the third inductance device L3 for thehigh-frequency pass and the other end thereof connected to the groundside; and two load output terminals X and Y connected to both ends ofthe second inductance device L2.

[0059] A typical bridge circuit will now be described with reference toFIG. 2. The typical bridge circuit includes a first switching device SW1for receiving direct current (DC) power; a second switching device SW2for receiving the DC power; a fourth switching device SW4 connected tothe first switching device SW1 through a path (A->B) between a firstnode A and a second node B; a third switching device SW3 connected tothe second switching device SW2 through a path (B->A) between the secondnode B and the first node A; and a second inductance device L2 arrangedin the path (A-B) between the first node A and the second node B.Operation of the bridge circuit will now be described. The switchingdevices SW1˜SW4 receive a switching control signal, respectively, andone pair of the first and fourth switching devices SW1 and SW4 and theother pair of the second and third switching devices SW2 and SW3alternately perform a turn-on/turn-off operation.

[0060] An output voltage value between the node A and the node B in thebridge circuit becomes zero if impedance values of the two switchingdevice pairs SW1 and SW4 and SW2 and SW3 symmetrical to each other arethe same. This bridge circuit theory was verified a long time ago.

[0061] As shown in FIG. 2, the D-class amplifier of the presentinvention is configured so that one pair of the first and fourthswitching devices SW1 and SW4 and the other pair of the second and thirdswitching devices SW2 and SW3 are symmetrical to each other.

[0062] A voltage distribution within the bridge circuit will now bedescribed in the case where the first and fourth switching devices SW1and SW4 are turned on. First, a total amount of electric current isexpressed as i=311V/(ZL1+ZL2+ZL3). Where voltages of the two capacitancedevices C1 and C2 arranged at both ends of the second inductance deviceL2 are buffered, EC1=i* (ZL2+ZL3) and EC2=i*ZL3. Furthermore, wherevoltages of the two capacitance devices C1 and C2 arranged at both endsof the second inductance device L2 are buffered when the second andthird switching devices SW2 and SW3 are turned on, EC1=i*ZL1 andEC2=i*(ZL2+ZL1). In this case, it is assumed that a turn-on time of thefirst and fourth switching devices SW1 and SW4 corresponding to the path(A->B) and a turn-on time of the second and third switching devices SW2and SW3 corresponding to the path (B->A) are the same as each other, andthe above-described equations are calculations when an ideal circuit isconfigured.

[0063] As seen above, the same level voltage is applied to the firstcapacitance device C1 and the second capacitance device C2, and zerovoltage due to the inverse polarity is applied to both ends of thesecond inductance device L2 in view of the second inductance device L2.

[0064]FIG. 3 is a graph illustrating the characteristics of boostedvoltage across a capacitance device varying with time.

[0065] As seen in FIG. 3, voltage of the capacitance device is boostedaccording to time lapse. It can be seen that a charging time of thecapacitance device is controlled and hence voltages at both ends of thecapacitance device can be controlled.

[0066] In other words, where capacitances of the first and secondcapacitances C1 and C2 are the same as each other, the voltages of thecapacitance devices become the same as each other, i.e., EC1=EC2, iftheir current on times are the same as each other. On the other hand,voltages of the capacitances become different from each other by adifference of the current on times when the current on times aredifferent from each other. For example, the current on time of the firstcapacitance device C1 is longer than that of the second capacitancedevice C2, it is concluded that EC1>EC2. A voltage difference indicatingEC1−EC2 is applied to both ends of the second capacitance device L2,i.e., the load output terminals X and Y. When charging times of thefirst and second capacitance devices C1 and C2 are controlled, voltagelevels and polarities associated with the load output terminals X and Ycan be controlled.

[0067] Operation of the above-described D-class amplifier for outputtingsine waves to the load output terminals X and Y in response to a sinewave generation control signal from a sine waveform controller will bedescribed with reference to FIGS. 4 and 5.

[0068]FIG. 4 is a view illustrating an operation for outputting sinewave power to a load; and FIG. 5 is a block diagram illustrating aconnection structure in the case where a circuit for generating a sinewave and applying the generated sine wave to the load is applied to amobile electronic product or backup equipment such as a UPS usingcommercial alternating current (AC) power. The above-described operationand connection structure will now be described with reference to FIGS. 4and 5 along with FIGS. 1 to 3.

[0069] A sine waveform controller 505 applies a sine wave generationcontrol signal to the switching devices SW1˜SW4, and performs a controloperation so that one pair of the first and fourth switching devices SW1and SW4 and the other pair of the second and third switching devices SW2and SW3 can alternately perform a turn-on/turn-off operation. Every timea predetermined switching period is shorter than a commercial AC powerperiod, the sine waveform controller 505 performs a control operation sothat a difference between a turn-on time of one pair of the switchingdevices and a turn-on time of the other pair of the switching devicescan be generated, and the output terminal X or Y can output voltage ofthe turn-on time difference. The sine waveform controller 505 adjuststhe turn-on time difference every time the switching period, andperforms a control operation so that the voltage outputted through theoutput terminal X or Y can be sine waveform voltage (e.g., an AC 220Voutput).

[0070] To output, to the load output terminal X or Y, output voltage{circle over (5)} of a waveform equal to the commercial AC power, thesine waveform controller 505 provides, to the switching devices SW1˜SW4,a sine wave generation signal for controlling a difference between aturn-on time t1 of one pair of the switching device SW1 and SW4 and aturn-on time t2 of the other pair of the switching devices SW2 and SW3every time a switching period (e.g., 50 KHz) is shorter than acommercial AC power period (e.g., 60 Hz).

[0071] For example, when a value of the switching period is “100”, avalue of the turn-on time t1 is “90”, and a value of the turn-on time t2is “10”, the load output terminal X or Y outputs an output voltagesignal corresponding to a time difference value “80” between the valueof the turn-on time t1 and the value of the turn-on time t2. On theother hand, when a value of the turn-on time t1 is “10” and a value ofthe turn-on time t2 is “90”, the load output terminal X or Y outputs anoutput voltage signal corresponding to a time difference value “80”. Thevoltage signals based on different polarities are outputted in theabove-described two cases. Furthermore, when both values of the turn-ontimes t1 and t2 are “50”, the output voltage becomes “0”.

[0072] When the output voltage {circle over (5)} is 2, 4, 6, 8, . . . ,96, 98, 98, 96, . . . , 4, 2, 0, −2, −4, . . . as shown in FIG. 4 everytime the switching period for generating one period of the commercial ACpower, the turn-on times t1 and t2 associated with a correspondingswitching period are determined. For example, when the output voltage ofa predetermined switching period is “2”, t1=51 and t2=49. Furthermore,the output voltage of a predetermined switching period is “−2”, t1=49and t2=51.

[0073] In other words, to provide the output voltage of a waveform equalto the commercial AC power, the sine waveform controller 505 maintains,during at least one commercial AC power period, the turn-on times t1 andt2 required for generating corresponding output voltage every time apredetermined switching period is shorter than a commercial AC powerperiod.

[0074] The sine waveform controller 505 outputs, to the first and fourthswitching devices SW1 and SW4, a sine wave generation control signalcorresponding to the turn-on time t1, and outputs, to the second andthird switching devices SW2 and SW3, a sine wave generation controlsignal corresponding to the turn-on time t2, in each switching period.

[0075] On the other hand, the D-class amplifier 503 can receive a DCvoltage signal stored in the battery (not shown), and directly receive aDC voltage signal from the battery. Alternatively, the D-class amplifier503 can receive a DC voltage signal boosted by the DC-DC converter 501as shown in FIG. 5. A detailed description of the DC-DC converter 501well known and disclosed in FIG. 1 of Korean Patent Publication No.278699 will be omitted.

[0076] Since the DC-DC converter 501 uses a switching transformationmethod rather than a linear transformation method as in the conventionalUPS, a volume and weight of the UPS can be considerably reduced when thesine wave generation circuit of the present invention is applied to theUPS.

[0077]FIG. 6 is a block diagram illustrating an exemplary UPS equippedwith the sine wave generation circuit for generating a sine wave andapplying the generated sine wave to a load in accordance with thepresent invention. The UPS will be described with reference to FIG. 6along with FIGS. 1 to 5.

[0078] System elements indicated by reference numerals 611, 613, 617 and619 associated with the D-class amplifier of the present invention shownin FIG. 6 have been described above in detail. System elements indicatedby reference numerals 601 to 609, 615, 621 and 623 configuring the UPSare well known. FIG. 6 exemplarily shows how to apply the D-classamplifier of the present invention to the UPS.

[0079] The UPS of the present invention is equipped with a D-classamplifier 611, a waveform controller 613 and a DC-DC converter 617subsequent to a battery 619 without use of the linear transformer andthe LC resonant circuit of the conventional UPS (shown in FIG. 1). TheUPS of the present invention directly generates a true sine wave from DCpower stored in the battery 619 through a switching method and appliesthe generated true sine wave to the load.

[0080] Thus, since the volume and weight of the UPS is small and light,the UPS can be appropriately implemented in a mobile electronic product.Furthermore, since the UPS can apply a correct sine wave to the load, aproduct associated with the UPS can be protected.

[0081] Operation of the UPS shown in FIG. 6 will now be described.

[0082] An AC input unit 601 connected to an input terminal of acommercial AC power supply receives AC power and then outputs thereceived AC power to a switching unit 603 and an auxiliary power supply(or rectifying/charging unit) 609. An AC error detector 607 detects anerror of the commercial AC power received from the input unit 601. Upondetecting an error of the commercial AC power, the AC error detector 607notifies the switching unit 603 of the error. The switching unit 603applies the commercial AC power from the AC input unit 601 to the loadthrough an AC output unit 605 in a normal mode. Furthermore, uponreceiving an error detection signal from the AC error detector 607, theswitching unit 603 performs a switching operation to output a sinewaveform power signal from a D-class amplifier (or a bridge circuit forsine wave generation) 611 to the load through the AC output unit 605.

[0083] The rectifying unit 609 rectifies the commercial AC power fromthe AC input unit 601, converts the rectified power into DC power, andprovides the DC power to the charging unit 609. The charging unit 609charges a battery 619 with the DC power. A DC-DC converter 617 boostsand/or drops the DC power inputted from the battery 619 by apredetermined level of the commercial AC power and then inputs theboosted or dropped DC power into the D-class amplifier 611. At thistime, an overcurrent suppressor 623 senses electric current generatedfrom the DC-DC converter 617 and outputs a control signal so thatovercurrent cannot occur.

[0084] As in the typical bridge circuit for power conversion, theD-class amplifier 611 includes a first inductance device L1 for highfrequency pass arranged on the path (A-L2) between the second inductancedevice L2, arranged on a path for connecting the node A and the node B,and the node A; the third inductance device L3 for high frequency passarranged on a path (L2-B) between the second inductance device L2 andthe second node B; the first capacitance device C1 including one endthereof connected to the third node C arranged on a path (L1-L2) betweenthe first inductance L1 and the second inductance device L2, and theother end thereof connected to the ground side; the second capacitancedevice C2 including one end thereof connected to the fourth node Darranged on a path (L2-L3) between the second inductance device L2 andthe third inductance device L3 for high frequency pass and the other endthereof connected to the ground side; and the two output terminals X andY connected to both ends of the second inductance device L2.

[0085] In order for the output terminals X and Y to produce outputvoltage of a waveform equal to the commercial AC power, a sine waveformcontroller 613 performs a control operation so that a difference betweena turn-on time t1 of one pair of the switching devices SW1 and SW4provided in the bridge circuit 611 for sine wave generation and aturn-on time t2 of the other pair of the switching devices SW2 and SW3provided in the bridge circuit 611 can be generated, and the outputterminal X or Y can output voltage of the turn-on time difference everytime a switching period is shorter than a commercial AC power period.The sine waveform controller 613 adjusts the turn-on time difference ineach switching period, and performs a control operation so that thevoltage outputted through the output terminal X or Y can be sinewaveform voltage.

[0086] As apparent from the above description, the present inventionenables a correct sine waveform to be directly generated through only acontrol operation for a D-class amplifier having a low-speed switchingperiod and a cheap microcomputer without use of a high-speedanalog/digital (A/D) converter, a switching device, a digital signalprocessor (DSP) and a digital/analog (D/A) converter, such that a sinewave generation circuit for directly generating a true sine wave from DCpower stored in a battery through a switching method and applying thegenerated true sine wave to a load and an uninterruptible power supplysystem (UPS) using the same can be appropriately implemented. Further,as an electronic product and backup equipment using the commercial powercan be miniaturized and lightened in weight, a mobile electronic productcan be implemented. Furthermore, as the true sine wave (or correct sinewaveform) can be directly generated through the switching method and thegenerated true sine wave can be applied to the load, a product can beappropriately protected.

[0087] In the UPS, the D-class amplifier can be easily applied to acircuit for converting the DC power stored in the battery into awaveform signal of the commercial AC power and applying the convertedpower to the load.

[0088] Since the present invention uses a switching transformationmethod without using a linear transformation method and an inductanceand capacitance (LC) resonant circuit provided in the conventional UPS,the UPS can be miniaturized and lightened in weight where the sine wavegeneration circuit is applied to the UPS.

[0089] Although the preferred embodiments of the present invention havebeen disclosed for illustrative purposes, those skilled in the art willappreciate that various modifications, additions and substitutions arepossible, without departing from the scope of the invention.Accordingly, the present invention is not limited to the above-describedembodiments, but the present invention is defined by the claims whichfollow, along with their full scope of equivalents.

What is claimed is:
 1. An uninterruptible power supply system (UPS),comprising: a rectifier for rectifying commercial alternating current(AC) power from an input terminal and converting the AC power intodirect current (DC) power; a charger for charging a battery with the DCpower; the battery for providing the DC power; a DC-DC converter forboosting and/or dropping the DC power inputted from the battery by apredetermined level of the AC power; a D-class amplifier for receivingthe DC power from the DC-DC converter and outputting a sine waveformpower signal in response to a waveform control signal; a sine waveformcontroller for controlling a sine waveform generation operation of theD-class amplifier; and a switching unit for switching the commercial ACpower from the input terminal to a load in a normal mode, and switchingthe sine waveform power signal from the D-class amplifier to the loadwhen an error of the commercial AC power is detected.
 2. The UPS as setforth in claim 1, wherein the D-class amplifier comprises a bridgecircuit for power conversion, the bridge circuit comprising: a firstinductance device (L1) for high-frequency pass arranged on a path (A-L2)between a second inductance device (L2), arranged on a path forconnecting a first node (A) and a second node (B), and the first node(A); a third inductance device (L3) for high-frequency pass arranged ona path (L2-B) between the second inductance device (L2) and the secondnode (B); a first capacitance device (C1) including one end thereofconnected to a third node (C) arranged on a path (L1-L2) between thefirst inductance device (L1) and the second inductance device (L2), andthe other end thereof connected to a ground side; a second capacitancedevice (C2) including one end thereof connected to a fourth node (D)arranged on a path (L2-L3) between the second inductance device (L2) andthe third inductance device (L3) and the other end thereof connected tothe ground side; and two load output terminals (X and Y) connected toboth ends of the second inductance device (L2), wherein the sinewaveform controller performs a control operation so that a differencebetween a turn-on time of one pair of switching devices (SW1 and SW4)provided in the D-class amplifier and a turn-on time of the other pairof the switching devices (SW2 and SW3) provided in the D-class amplifiercan be generated and an output terminal (X or Y) can output voltage ofthe turn-on time difference every time a predetermined switching periodis shorter than a commercial AC power period, and wherein the sinewaveform controller adjusts the turn-on time difference in eachswitching period, and performs a control operation so that the voltageoutputted through the output terminal (X or Y) corresponds to sinewaveform power equal to the commercial AC power.
 3. A sine wavegeneration circuit for converting a waveform of direct current (DC)power stored in a battery into a waveform of commercial alternatingcurrent (AC) power and applying the commercial AC power, comprising: abridge circuit for sine wave generation, the bridge circuit comprising:a first switching device (SW1) for receiving the DC power; a secondswitching device (SW2) for receiving the DC power; a fourth switchingdevice (SW4) connected to the first switching device (SW1) through apath (A->B) between a first node (A) and a second node (B); a thirdswitching device (SW3) connected to the second switching device (SW2)through a path (B->A) between the second node (B) and the first node(A); a second inductance device (L2) arranged in the path (A-B) betweenthe first node (A) and the second node (B); a first inductance device(L1) for high-frequency pass arranged in a path (A-L2) between the firstnode (A) and the second inductance device (L2); a third inductancedevice (L3) for high-frequency pass arranged in a path (L2-B) betweenthe second inductance device (L2) and the second node (B); a firstcapacitance device (C1) including one end thereof connected to a thirdnode (C) arranged on a path (L1-L2) between the first inductance device(L1) and the second inductance device (L2), and the other end thereofconnected to a ground side; a second capacitance device (C2) includingone end thereof connected to a fourth node (D) arranged on a path(L2-L3) between the second inductance device (L2) and the thirdinductance device (L3) and the other end thereof connected to the groundside; and two load output terminals (X and Y) connected to both ends ofthe second inductance device (L2); and a sine waveform controller forapplying a sine wave generation control signal to the switching devices(SW1˜SW4), and performing a control operation so that one pair of thefirst and fourth switching devices (SW1 and SW4) and the other pair ofthe second and third switching devices (SW2 and SW3) can alternatelyperform a turn-on/turn-off operation, wherein the sine waveformcontroller performs a control operation so that a difference between aturn-on time of one pair of switching devices and a turn-on time of theother pair of switching devices can be generated and an output terminal(X or Y) can output voltage of the turn-on time difference every time apredetermined switching period is shorter than a commercial AC powerperiod, and wherein the sine waveform controller adjusts the turn-ontime difference in each switching period, and performs a controloperation so that the voltage outputted through the output terminal (Xor Y) corresponds to sine waveform power equal to the commercial ACpower.
 4. The sine wave generation circuit as set forth in claim 3,further comprising: a DC-DC converter for boosting and/or dropping theDC power inputted from the battery by a predetermined level of the ACpower and inputting the boosted or dropped DC power into the bridgecircuit for sine wave generation.